Fix null watch time not respecting default

Preferences documentation had labeled "do not add to watchlist" as the default option for watch time, however it turns out that this is not the default: the default is whatever was set in the MediaWiki preferences. Even if the documentation was changed, however, setting the option to "0" still would have not been able to disable watchlisting.

This merge request solves the documentation errors and the configuration reading issues by requiring a non-null value for rwWatchTime in order to make watchlist changes, and allows delisting pages from the watchlist if the user desires.